[detaro]

"As fast as C" would commonly be interpreted as "a program written in it will be as fast as a well-written C equivalent", not as "there is a C program with the same performance characteristics".

That a language is compiled to C does not mean that its compiler is going to be able to produce a C program that's as good as a that well-written C equivalent. (A relatively obvious example would be a compiler that introduces a heavy runtime, and doesn't give the C compiler enough information for it to get rid of the runtime)

It's the same with assembly code: that a compiler produces assembly does not mean the resulting program is fast.

[19ylram49]

> "As fast as C" would commonly be interpreted as "a program written in it will be as fast as a well-written C equivalent"

That’s your interpretation, which is fine, but the objective meaning stands. Even the idea of “well-written C” is, in my experience, fairly subjective amongst C programmers.

[ezrast]

Remember that we're comparing languages, not programs. A language can be thought of as the space of all programs that can be interpreted by that language. In any language, any algorithm can be implemented arbitrarily slowly. So the only meaningful point of comparison between language X and language Y is the upper bound on the performance of each language's program-space.

That some particular C program exists that is at least as slow as a program in some other language is always true, trivially, and so is not a good interpretation of "as fast as C" regardless of its objectivity.

[RussianCow]

You're missing the point. The fact that a language compiles down to C doesn't mean it compiles down to efficient C. At the simplest level, the compiled code could add a bunch of unnecessary function calls and pointers and other forms of indirection that wouldn't be present in hand-written C. But for a more extreme example, you could also compile an interpreter or VM to C, and it would still be much slower than the equivalent hand-written C code. This is why "as fast as C" typically refers to normal, hand-written C code—even though there is no formal definition for what "normal C" looks like, it's still a useful description.

[19ylram49]

> The fact that a language compiles down to C doesn't mean it compiles down to efficient C.

Where do you see me claiming otherwise?

> At the simplest level, the compiled code could add a bunch of unnecessary function calls and pointers and other forms of indirection that wouldn't be present in hand-written C.

Again, why are you telling me this? Please quote where I claimed otherwise.

> But for a more extreme example, you could also compile an interpreter or VM to C, and it would still be much slower than the equivalent hand-written C code.

The more that I read your response, the more that it seems that you’re debating yourself, because I’m not sure why you’re telling me this. You started your response by telling me that I’m “missing the point” when, in reality, you seem to have not even read my point. My main point was the following:

> If the code from which the resulting machine code is compiled is C, then it’s objectively “as fast as C” […] your resulting program will perform as fast as a C compiler [worth its salt] can get you.

This is true. I made no claims re efficiency; “as fast as C” and “as fast as efficient hand-written C” aren’t interchangeable claims. Forgive me for not assuming efficiency, because I’ve seen a good amount of inefficient hand-written C code in my years.

> This is why "as fast as C" typically refers to normal, hand-written C code—even though there is no formal definition for what "normal C" looks like, it's still a useful description.

Says who though? I’m professionally experienced in C, and as is very clear by this discussion, it’s down to individual interpretations.

[RussianCow]

But that's not at all a useful way to describe a language. Why would someone ever describe a language as "as fast as C" and not mean "as fast as typical hand-written C"? What would be the purpose of that? With your interpretation, CPython is "as fast as C", since it's written in C, and yet that's not a claim anyone would actually make.